Prior to the invention of the transistor, vacuum tube triodes were the principal three-terminal devices used in active electronic circuits. The vacuum tube triode comprises three electrodes:
(i) A cathode where electrons are emitted by thermionic emission; PA1 (ii) An anode where the electrons are collected; PA1 (iii) An open grid placed in the vacuum tube between the cathode and the anode. Typically a voltage is applied between the anode and the cathode with the anode positive with respect to the cathode such that current flows from anode to cathode (electrons flow from cathode to anode). A bias voltage is applied to the grid with respect to the cathode; typically a high impedance voltage source is used so that the grid does not draw significant current. In this way, a relatively small grid-cathode voltage can be used to control the current between the cathode and the anode, a key feature in the use of such triodes in amplifier circuits, oscillator circuits, control circuits and the like. PA1 (a) Ever smaller devices, for example with sub-micron resolution, thereby achieving more circuits per unit area and higher frequency response; PA1 (b) Higher mobility materials so as to reduce the time required for the carriers to travel from source to drain. PA1 i) Emission of light at the desired brightness levels often requires the application of a relatively high voltage. PA1 ii) The external conversion efficiency is low. PA1 iii) Brightness is limited, particularly at voltages below 5 volts.
The transistor, in its various forms, is also a three-terminal device; a solid state analogue of the triode. The transistor does not utilize a grid to control the current in the external circuit; such control is obtained by other means. For example, in a field effect transistor (FET), the source-drain current is controlled by the gate voltage; application of a gate voltage creates carriers in the source-to-drain channel and renders the channel conducting. Thus, an FET is an active three-terminal device, the gate voltage controls the source-drain current. An npn transistor is, fundamentally, two pn junction diodes back-to-back. Again, an npn transistor is a three-terminal device; by controlling the bias of the two diodes independently through contact to the center p-type layer, one can control the current between the two n-type layers and thereby achieve gain (S. M. Sze, Physics of Semiconductor Devices (Wiley, N.Y., 1981)).
In the FET, as well as in transistors in other forms, the frequency response is limited by the size of the device. For example, FETs with large source-to-drain spacing will be slow relative to FETs with a smaller source-to-drain spacing. The response is limited by the time required for the carriers to travel from source to drain. Thus, modern semiconductor technology strives for the following:
It is well known that there is considerable cost and technical difficulty involved in the fabrication of such sub-micron transistors and arrays of such transistors (integrated circuits) using high mobility single crystal materials.
The semiconductor on-chip technology that is the heart of modern electronics is not appropriate to certain important electronic applications. An example is the Active Matrix Liquid Crystal Display. In order to achieve the desired contrast in a liquid crystal display, it is desirable to have each pixel of the display is controlled by a transistor. In the Active Matrix Liquid Crystal Display, it is advantageous to fabricate these control transistors as thin film transistors on a surface which is an integral part of the display. The technology is limited by poor yields, high costs and poor high frequency response (low speed operation), all of which result from the need to use conventional transistor design with the low mobility materials amenable to thin film deposition on surfaces (for example, amorphous silicon, organic semiconductors and the like).
Thus, there is a need for improved low cost three-terminal solid state devices, analogous to the vacuum tube triode and to the transistor, which can be easily manufactured, and which utilize low mobility materials in a configuration where the cathode to anode distances are submicron.
There is also a need for new concepts in the area of electroluminescent devices. Recently, light-emitting diodes (LEDs) fabricated with conducting polymers (H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns and A. B. Holmes, Nature 347, 539 (1990); D. Braun and A. J. Heeger, Appl. Phys. Lett. 58, 1982 (1991)) have attracted attention due to their potential for use in display technology. Such LED's are layered structures with two electrodes separated by charge transporting and luminescent layers. Among the promising materials for use in polymer LEDs are poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene), MEH-PPV, a semiconducting polymer with energy gap E.sub.g .about.2.1 eV (U.S. Pat. No. 5,189,136) and poly(2,5-bis(cholestanoxy)-1,4-phenylene vinylene) (BCHA-PPV), a semiconducting polymer with energy gap E.sub.g .about.2.2 eV (C.i.P. of U.S. Pat. No. 5,189,136, filed Mar. 8, 1993). Both MEH-PPV and BCHA-PPV are soluble in common organic solvents, thereby enabling relatively simple device fabrication by processing the active layer from solution. In the polymer LEDs the semiconducting polymer is sandwiched between two electrodes; a low-work function metal (such as Ca, Al, and the like); and a transparent electrode, such as indium/tin oxide (ITO) (D. Braun and A. J. Heeger, Appl. Phys. Lett. 58, 1982 (1991)). By using a conducting polymer, such as polyaniline (PANI) (rather than indium/tin-oxide, ITO) as the hole-injecting contact, robust and mechanically flexible LEDs have been prepared (G. Gustafsson, Y. Cao, G. M. Treacy, F. Klavetter, N. Colaneri, and A. J. Heeger Nature, 357, 477 (1992)).
Electroluminescent (EL) devices utilizing small organic molecules as the luminescent semiconducting species were well known and extensively developed prior to the discovery of light-emitting diodes made with conjugated polymers. See, for example, the following: C. W. Tang, S. A. Van Syke, Appl. Phys. Lett. 51, 913 (1987); C. W. Tang, S. A. Van Syke and C. H. Chen, J. Appl. Phys. 65, 3610 (1989); C. Adachi, S. Tokito, T. Tetsui and S. Saito, Appl. Phys. Lett. 55, 1489 (1989); C. Adachi, S. Tokito, T. Tetsui and S. Saito, Appl. Phys. Lett. 56, 799 (1989); M. Nohara, M. Hasegawa, C. Hosohawa, H. Tokailin, T. Kusomoto, Chem. Lett. 189 (1990). In the small molecule EL devices of the prior art, the active layers are typically deposited using vacuum deposition. Transport layers (both electron transport layers and hole transport layers) were developed to improve the carrier injection and to balance the injection of electrons and holes so as to achieve higher efficiency.
Unfortunately, EL devices made with organic and/or polymeric materials as the semiconducting and luminescent materials suffer three important drawbacks that impede large-scale applications:
Recent progress has shown that by generalizing to heterojunction devices which include an electron transport layer, device efficiency can be somewhat improved in polymer LEDs using, for example, BCHA-PPV (C. Zhang, S. Hoger, K. Pakbaz, F. Wudl and A. J. Heeger, J. Electron. Mater. 22, 745 (1993)) or using a soluble cyano-derivative of poly(phenylene vinylene), PPV (N.C. Greenham, S. C. Moratti, D. D. C. Bradley, R. H. Friend, A. B. Holmes, Nature, 365, 628 (1993)).
Nevertheless, general and broad needs still exist for device concepts that result in light-emitting structures with increased efficiency, decreased turn-on voltage, and increased brightness.